1. Field of the Invention
The present invention relates to an organic electro luminescence device, and more particularly, to an organic electro luminescence device in which a cathode is provided with first to fourth electrodes.
2. Discussion of the Related Art
Recently, with a tendency for a display to be large-sized, there is an increasing demand for flat-panel displays occupying less space. One of these flat-panel displays is an organic electro luminescence device (hereinafter, referred to as an “organic EL device”) and a technology of the EL device has been rapidly developed. Several prototype products have already been demonstrated.
The organic EL device includes an anode substrate formed of a transparent electrode such as indium tin oxide (ITO), a cathode made of metal (Ca, Li, Al:Li, Mg:Ag, etc) having low work function, and a thin organic layer formed between the anode substrate and the cathode. If a forward voltage is applied to such an organic EL device, holes and electrons are injected at the anode substrate and the cathode, respectively. The injected holes and electrons are combined with each other to form excitons. The excitons provide a radiative recombination, which is called an electro luminescence phenomenon.
Here, material for the thin organic layer can be classified into low polymer material or high polymer material. In the case of the low polymer material, the thin organic layer is formed on a substrate using a vapor deposition process, and in the case of the high polymer material, it is formed on a substrate using a spin coating process. In order to operate elements at a low voltage, the thin organic layer is formed very thinly (about 1000 Å thick). The thin organic layer should be uniform and should not have defects such as a pin hole.
Additionally, although the thin organic layer can be formed of a single material, it is generally formed in multi-layer structure of several organic materials.
The reason for forming the organic device in a multi-layer structure is that the holes and the electrons can be effectively transferred to an organic light-emitting layer (EML) when using a hole transport layer (HTL) and an electron transport layer (ETL), since mobility of the hole is greatly different from that of the electron in the organic material.
If hole density and electron density are balanced in the organic light-emitting layer (EML), luminous efficiency is increased.
Additionally, in some cases, an energy barrier of the hole injection can be reduced if a hole injection layer (HIL) made of material such as conductive polymer is further formed between the anode substrate and the hole transport layer. An energy barrier of the electron injection can be also reduced if a buffer layer (an electron injection layer (EIL)), such as LiF layer of about 5 Å to 10 Å thick, is further formed between the cathode and the electron transport layer, such that luminous efficiency is enhanced and a driving voltage is reduced.
In the organic EL device, organic material for the thin organic layer inserted between both electrodes has an advantage in that various types of material can be easily synthesized because of a simple synthesis path and color tuning is possible. The organic material is classified into low polymer material and high polymer material.
In case the thin organic layer is formed of the low polymer material, a driving voltage is lowered and an advantage of a thin layer about 100 nm thick is obtained. Also, it is possible to obtain a high resolution and reproduce a natural color. Meanwhile, in case the thin organic layer is formed of the high polymer material, thermal stability and a low driving voltage, as well as a flexible characteristic, can be obtained, and a large-sized device can be manufactured at a low cost. Also, a polymer chain of one dimension is arranged to emit polarized light and an on-off speed is fast.
Further, the organic EL device has a cathode made of alkali metal having an excellent electron injection characteristic to enhance brightness of the organic EL device and employs material with high luminous efficiency as material for the organic light-emitting layer.
However, the organic EL device according to the related art has a disadvantage that so-called dark spots frequently appear on a display panel. A dark spot constitutes an electro non-luminescence region caused by degradation of the material characteristics of the thin organic layer and/or a stripping of the cathode.
The degradation of material characteristic of the thin organic layer and/or the stripping of the cathode are/is caused due to an existence of oxygen (O2) or moisture (H2O) contained in air around the organic EL device. This causes a degradation of luminescence characteristic such as brightness and chromaticity of the organic EL device.
Several measures have been taken so as to resolve these problems. One of them is to remove moisture contained in air around the organic EL device using an absorbent provided in an inner space of the organic EL device.
In more detail, in order to efficiently remove the moisture contained in air around the sealed space of the organic EL device, the absorbent is placed to face the cathode in the sealed space.
FIG. 1 is a sectional view of an organic EL device according to the related art.
Referring to FIG. 1, the organic EL device according to the related art includes an anode (a transparent electrode) substrate 12 formed of transparent ITO, a thin organic layer 14 and a cathode 16 that are sequentially deposited on the transparent electrode substrate 12. As described above, the thin organic layer 14 includes a hole injection/transport layer, an organic light-emitting layer and an electron injection/transport layer.
Additionally, in order to prevent the degradation of the luminescence characteristics of the thin organic layer 14 and the stripping of the cathode, an absorbent 20 for removing moisture is fixedly adhered to an inner surface of a sealing member 18 defining an inner space.
Meanwhile, a protection layer (not shown) can be formed to prevent oxygen or moisture from permeating the thin organic layer 14 or the cathode 16, and the protection layer includes the absorbent or other materials suitable to absorb or consume oxygen or moisture. Additionally, the protection layer can be stacked on outer surfaces of the anode substrate 12, the thin organic layer 14 and the cathode 16 of the organic EL device.
The cathode 16 may be generally formed with one kind of metal. Meanwhile, in order to enhance luminous efficiency, the cathode 16 may be formed with two electrodes. In other words, a first electrode is formed of a metal having a low work function and then a second electrode acting as a protection metal layer is formed on the first electrode to protect the first electrode.
However, in case the first electrode is formed of a metal having a low work function and then the second electrode acting as a protection metal layer is formed on the first electrode to protect the first electrode, oxygen or hydrogen may permeate the second electrode due to strong reactivity of the first electrode with respect to oxygen or moisture, and then react with the first electrode. In some cases, oxygen or moisture may permeate an interfacial surface of the thin organic layer.